This invention relates to a method for providing conductive paths from the outer surface of conductor insulating jackets to the magnetic cores in dynamoelectric machines for purpose of inhibiting corona in the machine.
In a laminated magnetic core for a dynamoelectric machine the dimensions of the teeth vary somewhat between laminations, and the positions of the laminations vary in the core stacks. These irregularities are great enough that the surfaces of the slots have somewhat jagged faces. The coils used in the machine are insulated with outer jackets consisting of wrappings of porous materials impregnated with certain thermosetting resins and shaped in a mold while the resin is cured to a solid and hard state. This leaves the outer surfaces of the coils very smooth, hard and with some irregularities in their flatness. When these coils are in place in the slots, the smooth outer surfaces of the coils make physical contact with some of the high laminations, leaving voids between the jacket and other laminations.
Variations in coil side dimensions can lead to looseness of sides in their slots, resulting in voids or exaggerating the voids mentioned above. The tolerance of a coil side may be in a range of several mills. It is known to insert packing strips between a coil side and a slot wall to tighten-up the fit and thereby prevent movement of the side in the slot. These strips may be thin, non-metallic, electrically conductive springs which secure the side in the slot and provide electrical paths of controlled resistance between the coil armor and the slot wall. However, since the strips come in discrete thicknesses that can be driven between a coil side and a slot wall, this packing may not always make a side a tight fit in its slot; any looseness may lead to coil movement resulting in corona problems.
Electrical grade resinous materials should be good insulators of electricity and reasonably good conductors of heat. Certain epoxy resins meet this specification. However, those that do meet the specification cure to a hard state, and once fully cured, they do not soften appreciably when reheated during operation of the machine. These materials produce the so-called hard-bar windings in which the resin impregnants do not soften when the coils become hot and flow into the voids as did the asphaltic impregnants that preceded them. Because the resinous materials do not soften with heat and flow into the voids, the voids remain.
Initially, the armor covering on the coils usually make good electrical contact with many of the laminations defining the slot walls. These contacts placed the armor and core at essentially the same potential. However, vibration from machine operation will often break these contacts and cause sufficient coil movement to lead to a difference of potential between the armor and core. This potential difference imposes electrical stresses on the air in the voids formed at the breaks, stresses that may well be great enough to cause partial discharge from the coil surfaces to the core, i.e., a phenomenon often referred to as corona or corona discharge. The improved resinous materials make higher operating voltages possible, and this in turn subjects the void regions to high electrical stresses, or these newer insulations may even increase stresses without an increase in voltage. It is well known that in the presence of corona discharge insulating materials are eroded and may eventually break down.
Our Canadian Pat. No. 1,016,586, issued Aug. 30, 1977 and entitled "Grounding of Outer Winding Insulation to Cores in Dynamoelectric Machines," describes and claims a means for inhibiting corona in dynamoelectric machines such as large power generators. In this application, an elastomeric material of controlled electrical resistance is applied to the coil sides and then cured before the sides are inserted into the slots in the core. The lay of this material on a coil side is such that the material deforms as the side is inserted into a slot, causing the material to make contact with the laminations. In this particular approach to the corona problem, the elastomeric material is applied to the coil sides before they are inserted in the slots; the material cannot be applied to the coil sides already in place in the slots.
U.S. Pat. No. 3,824,683 issued July 23, 1974, Rhudy, discloses a method for reducing corona in machines having the coils in place in the slots of the core, for example, treating a machine that has been in service. In this particular method, a free flowing, electrically conductive paint is made to flow in between the coil sides and the slot walls so as to coat both. After the paint is dry, an elastomeric material is deposited in some of the air ducts in the core in contact with the coil surfaces and core. This material contains an electrically conductive filler whereby conductive paths are provided between coils and core.
The object of this invention is to improve the inhibition of corona in dynamoelectric machines having the coils in place in the slots of the magnetic core.
According to the invention conductive paths are formed between the winding and the core of a dynamoelectric machine by injecting a viscous, semi-conducting, elastomeric material between the coil sides and the slot walls by way of the air ducts in the core, and thereafter curing the material. The cured material is a tough rubber-like substance of an electrical resistance high enough not to short circuit the laminations of the core and yet low enough to conduct electric charge from the coil armor to the core; it is a substance that is capable of retaining its strength, elasticity, conductivity, etc., and remaining in place between the coils and core under vibration, coolant flow, electric stresses, repeated temperature changes, etc., for the normal operating life of the machine. These paths conduct electric charge from the coils to the core, and thereby inhibit the formation of corona.
Certain silicone resins are well suited for use as conductive path forming materials between the coils and core. Inherently, silicone resins are good electrical insulators, but some are relatively good conductors of heat as well. The good heat conductors are preferred because they will transfer heat from the coils to the core. To make them electrically conductive for purposes of conducting electric charge, they are filled with conductive fine particle materials such as carbon powder, lamp black or a mixture thereof. The amount of conductive powder added to the resin is just enough to give the cured product the necessary electrical properties, but not enough to detract significantly from its physical properties.
Apparatus for injecting an uncured elastomeric material into the space between a coil side and a wall of the slot containing the side consist of an injector tool adapted for insertion into a selected air duct in the core into communication with the coil side for material flow from the tool into the space between the side and slot wall; means for securing the tool in the duct while material flow takes place; and means for forcing material flow from the tool into the space. In a preferred apparatus, the securing means and the flow forcing means may be pneumatic actuating means .